In chronic myelogenous leukemia (CML), treatment of early stage disease has been significantly improved with the discovery of imatinib mesylate (IM, Gleevec), but with the progression of the disease, drug resistance develops which is due to mutation of the ATP binding site domain within the Bcr-Abl tyrosine kinase, the activation of Lyn kinase (a Src-related tyrosine kinase) and other mutations that produce a more aggressive disease. Thus, patients relapse and the disease progresses to blast crisis (BC) stage where survival is limited to only several months. No suitable drug treatment for BC is available, and the mechanism of drug resistance for BC is unclear. A new synthesized small molecule, WP1193, a caffeic acid derivative, induced apoptosis in all types of drug-resistant CML cells including IM-resistant T315I BCR-ABL+ cells, IM-resistant CML cells with up-regulated Lyn tyrosine kinase (K562-R) and cells from blast crisis CML patients. Treatment of BCR-ABL+ cells with WP1193 inhibited Janus kinase activities, leading to strong reduction of Bcr-Abl; this treatment also reduced STAT3 and pTyr 705 STAT3 protein levels. CML cell lines contain a large signaling network complex composed of Bcr-Abl, Jak2, HSP90 and its client proteins. HSP90 is a molecular chaperone that plays an important role in conformational maturation and stabilization of client proteins (Bcr- Abl, Jak2, STAT3, etc.), which are physically associated in the signaling network complex. This network complex (estimated to be between 2–5 million daltons) has been characterized by gel filtration column chromatography. This complex is disassembled by treatment of Bcr-Abl+ cells with WP1193 (10 μM for 3–6 h). Treatment of the cells with WP1193 also inhibited binding of STAT3 to its consensus DNA sequence and reduced levels of HSP90α transcripts, HSP90α protein and its client proteins, which include Bcr-Abl, Jak2 and Akt. WP1193 also reduced solid tumors in mice inoculated with IM-resistant K562-R cells, and inhibited leukemic effects caused by T315I BCR-ABL+ cells. These results indicate that disruption of the Bcr-Abl/Jak2/HSP90 network complex by WP1193 overcomes drug resistance in advanced stage CML cells. Since disruption of this network complex causes apoptosis induction in all IM-resistant and late stage CML patient cells tested so far, and since no significant toxicity was observed in short-term mouse experiments, WP1193 may have medical utility for the treatment of IM-resistant forms of CML and late stage CML where other drugs fail. One novel feature of WP1193 is its ability to reduce the activity of several important components of the network, namely Bcr-Abl, Jak2 and STAT3 activities and reduction of HSP90α, which then causes a high level of apoptosis in IM-resistant cells and late-stage CML cells.

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